A New Milky Way Dwarf Galaxy in Ursa Major

In this Letter, we report the discovery of a new dwarf satellite to the Milky Way, located at (α2000, δ2000) = (15872, 5192) in the constellation of Ursa Major. This object was detected as an overdensity of red, resolved stars in Sloan Digital Sky Survey data. The color-magnitude diagram of the Ursa Major dwarf looks remarkably similar to that of Sextans, the lowest surface brightness Milky Way companion known, but with approximately an order of magnitude fewer stars. Deeper follow-up imaging confirms that this object has an old and metal-poor stellar population and is ~100 kpc away. We roughly estimate MV = -6.75 and r1/2 = 250 pc for this dwarf. Its luminosity is several times fainter than the faintest known Milky Way dwarf. However, its physical size is typical for dwarf spheroidal galaxies. Even though its absolute magnitude and size are presently quite uncertain, Ursa Major is likely the lowest luminosity and lowest surface brightness galaxy yet known.

[1]  M. Irwin,et al.  A dwarf satellite galaxy in Sagittarius , 1994, Nature.

[2]  Peter B. Stetson,et al.  THE CENTER OF THE CORE-CUSP GLOBULAR CLUSTER M15: CFHT AND HST OBSERVATIONS, ALLFRAME REDUCTIONS , 1994 .

[3]  William E. Harris,et al.  A Catalog of Parameters for Globular Clusters in the Milky Way , 1996 .

[4]  M. Fukugita,et al.  The Sloan Digital Sky Survey Photometric System , 1996 .

[5]  et al,et al.  The Sloan Digital Sky Survey Photometric Camera , 1998, astro-ph/9809085.

[6]  Mario Mateo,et al.  DWARF GALAXIES OF THE LOCAL GROUP , 1998, astro-ph/9810070.

[7]  George Lake,et al.  Dark Matter Substructure within Galactic Halos , 1999, astro-ph/9907411.

[8]  Francisco Prada,et al.  Where Are the Missing Galactic Satellites? , 1999, astro-ph/9901240.

[9]  E. al.,et al.  The Sloan Digital Sky Survey: Technical summary , 2000, astro-ph/0006396.

[10]  J. Gunn,et al.  A Photometricity and Extinction Monitor at the Apache Point Observatory , 2001, astro-ph/0106511.

[11]  John E. Davis,et al.  Sloan Digital Sky Survey: Early Data Release , 2002 .

[12]  Heather A. Rave,et al.  The Ghost of Sagittarius and Lumps in the Halo of the Milky Way , 2001, astro-ph/0111095.

[13]  An SDSS Survey For Resolved Milky Way Satellite Galaxies. I. Detection Limits , 2001, astro-ph/0111025.

[14]  Christopher Palma,et al.  On the Distribution of Orbital Poles of Milky Way Satellites , 2001, astro-ph/0108474.

[15]  R. Lupton,et al.  Astrometric Calibration of the Sloan Digital Sky Survey , 2002, astro-ph/0211375.

[16]  D. Lamb,et al.  A Low-Latitude Halo Stream around the Milky Way , 2003, astro-ph/0301029.

[17]  Deep Wide-Field BVI CCD Photometry of the Sextans Dwarf Spheroidal Galaxy , 2003, astro-ph/0308469.

[18]  S. Majewski,et al.  Tracing the Galactic Anticenter Stellar Stream with 2MASS M Giants , 2003 .

[19]  M. F. Skrutskie,et al.  A Two Micron All Sky Survey View of the Sagittarius Dwarf Galaxy. I. Morphology of the Sagittarius Core and Tidal Arms , 2003, astro-ph/0304198.

[20]  Zeljko Ivezic,et al.  Andromeda IX: A New Dwarf Spheroidal Satellite of M31 , 2004 .

[21]  B. Willman,et al.  The observed and predicted spatial distribution of Milky Way satellite galaxies , 2004 .

[22]  E. K. Grebel,et al.  Theoretical isochrones in several photometric systems. II. The Sloan Digital Sky Survey ugriz system , 2004 .

[23]  S. Majewski,et al.  Exploring Halo Substructure with Giant Stars: A Diffuse Star Cloud or Tidal Debris around the Milky Way in Triangulum-Andromeda , 2004, astro-ph/0405437.

[24]  The Canis Major dwarf galaxy as the progenitor of the Monoceros tidal stream , 2005, Proceedings of the International Astronomical Union.

[25]  A. Klypin,et al.  The Anisotropic Distribution of Galactic Satellites , 2005, astro-ph/0502496.

[26]  Andrew A. West,et al.  A New Milky Way Companion: Unusual Globular Cluster or Extreme Dwarf Satellite? , 2004, astro-ph/0410416.